Engines may use various forms of fuel delivery to provide a desired amount of fuel for combustion in each cylinder. One type of fuel delivery uses a port injector for each cylinder to deliver fuel to respective cylinders. Still another type of fuel delivery uses a direct injector for each cylinder.
Further, engines have also been described using more than one injector to provide fuel to a single cylinder in an attempt to improve engine performance. Specifically, in U.S. 2005/0155578 an engine is described using a port fuel injector and a direct injector in each cylinder of the engine.
Another approach utilizing multiple injection locations for different fuel types is described in the papers titled “Calculations of Knock Suppression in Highly Turbocharged Gasoline/Ethanol Engines Using Direct Ethanol Injection” and “Direct Injection Ethanol Boosted Gasoline Engine: Biofuel Leveraging for Cost Effective Reduction of Oil Dependence and CO2 Emissions” by Heywood et al. Specifically, the Heywood et al. papers describes directly injecting ethanol to improve charge cooling effects, while relying on port injected gasoline for providing the majority of combusted fuel over a drive cycle.
However, the inventors herein have recognized a problem with such approaches. For example, a minimum valve opening amount, or minimum pulsewidth, may be encountered when varying injection of one or both injectors in response to conditions such as knock sensor feedback during engine operation, thus generating air-fuel errors.
Thus, in one approach, a method is provided for an engine configured to deliver an actively varying relative ratio of a first and second fuel via a delivery system having a first and second injector for a cylinder of the engine, where the first fuel has a lesser relative amount of alcohol. The method comprises varying an amount of injection from the first and second injector with operating conditions; and when said variation causes said first injector to approach a first minimum opening condition, increasing injection of said second injector and disabling said first injector, and when said variation causes said second injector to approach a second minimum opening condition, avoiding further reduction of said second injector and decreasing injection of said first injector.
In this way, it is possible to err on the side of providing extra fuel having a greater alcohol content (and thus a greater knock avoidance effect. For example, when the first injector approaches as minimum opening, it is disabled so that extra fuel from the second injector can be used to provide a desired total fuel injection amount. Likewise, when the second injector approach as minimum opening, further reduction is avoided so that at least the desired minimum amount of the second fuel is provided, and the first injector may be reduced if necessary to provide a desired total fuel injection amount.